In optical communication technologies, studies are in progress on a technologies which implement high-speed and large-capacity optical transmission systems achieving, for example, 100 gigabit/second (Gbps) or more. As examples of such technologies, orthogonal Frequency division multiplexing (OFDM) and Nyquist wavelength division multiplexing (WDM) are known.
In the OFDM, multiple orthogonal optical signals are made orthogonal to each other to reduce inter-signal interferences to the minimum, so that a frequency interval (in other words, wavelength interval) between the optical signals may be narrowed.
Meanwhile, in the Nyquist WDM, transmission data signals to be wavelength-multiplexed are formed into a particular waveform shape (for example, rectangular shape) by using, for example, Nyquist filtering by digital signal processing, so that a wavelength interval in the WDM optical signal may be narrowed.
The use of these technologies makes it possible to form a WDM optical signal super-dense in the wavelength interval, and thereby to improve the frequency usage efficiency in an optical transmission band (may also be referred to as “system band”) usable for an optical transmission system.
A technology of the related art is disclosed in Japanese Laid-open Patent Publication No. 09-261205.
With use of a super-dense WDM optical signal, the number of wavelengths (may be referred to as “channels”) allocated in an optical transmission band is expected to increase. On the other hand, as for an optical transmission apparatus (may be referred to as “station” or “node”) which transmits WDM optical signals, optical power that an optical amplifier provided therein is not infinite. In other words, the output optical power from the optical amplifier is limited.
For this reason, even if the number of channels in a system band is increased using the technology such as the OFDM or the Nyquist WDM which may increase the frequency usage efficiency, a transmittable distance of the WDM optical signal may be limited due to insufficiency of the output optical power from the optical amplifier.
The conventional WDM transmission technology may merely perform a pre-emphasis control of transmission optical power for channels on a transmission node so as to equalize optical signal to noise ratios (OSNRs) of optical signals of multiple channels included in a WDM optical signal received by a reception node.
Accordingly, this technology may fail to optimize the transmission optical power of the optical transmission apparatus provided with the optical amplifier by considering that the output optical power from the optical amplifier may be limited. As a result, constraints occur in the increase in efficiency of the optical transmission. For example, a constraint to the optical transmission distance occurs or a constraint to the number of transmittable channels (may also be referred to as “the number of stored channels”) occurs in some cases.